The stability characteristics of swirl-stabilized hydrogen-enriched CH4–O2–CO2 premixed flames were experimentally investigated to determine the effects of hydrogen addition and inlet velocity on flame stability under stoichiometric conditions (φ = 1.0). The stability limits in terms of blowout and flashback were identified over wide ranges of hydrogen fraction (HF: %H2 in H2–CH4 mixture) and oxygen fraction (OF: %O2 in O2–CO2 mixture) at fixed inlet velocity of 6 m/s. The lines of stability limits were plotted against the contours of adiabatic flame temperature (AFT), power density (PD), and Reynolds number (Re) to understand the physics behind the flame extinction mechanism. The effects of inlet velocity on flame stability limits under hydrogen enrichment were investigated by comparing the stability maps of the combustor for different inlet velocities, namely, 4.4, 5.2, and 6.0 m/s. The results show a stable combustion zone in the OF– HF space in the ranges of OF from 16 up to 44% and HF from 0 (pure CH4) up to 90%. However, increasing HF restricts the range of both OF and mixture Re for stable flame operation. Flame shapes at different inlet velocities were captured using a high-definition camera and compared to investigate the effects of OF and HF on flame stability. Flame visualizations near flashback and blowout limits were recorded to explore the physics behind flame extinctions mechanisms. The effect of reaction kinetic rates on the flame stability was investigated by recording flame shapes at fixed adiabatic flame temperature. The results show that the flames become gradually shorter and more compact with the increase in HF because of the enhanced reaction rates within the combustion zone. Insignificant changes in the flame shape were observed at fixed AFT or fixed Re operation.

The main challenge of improving spark ignition (SI) engines to achieve ever increasing thermal efficiencies and near-zero pollutant emissions today concerns developing turbulent combustion under homogeneous ultra-lean premixed mixtures (HULP). This continuous shift of the lean operation limit entails questions on the applicability limits of the combustion models used to date for SI engine design and optimization. In this work, an assessment of flamelet-based models, widely used in RANS SI engines simulations of premixed turbulent combustion, is carried out using an open-source 3D-CFD platform to clarify the applicability limits on HULP mixtures. Two different consolidated approaches are selected: the Coherent Flame Model (CFM) and the Flame Area Model (FAM). Both methodologies are embedded by the authors into the same numerical structure and compared against measurements over a simplified and controlled flame configuration, which is representative of engine-like conditions. The experimental steady-state flame of type “A” of the Darmstadt Turbulent Stratified Flame (TSF) burner is selected for the assessment. This configuration is characterized by flame measurements over a strong shear and mixing layer between the central high-speed CH4-air jet and the surrounding slow air co-flow, hence, it represents an interesting controlled condition to study turbulent HULP mixtures. A comparison between computed results and experimental data on trends of mean flow velocity, turbulence, temperature and mixture stratification was carried out. This enabled us to assess that the investigated flamelet-based combustion models failed in providing accurate and reliable results when the flame approaches turbulent HULP mixture conditions, demonstrating the urgency to develop models able to fill this gap.

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